1. Field of the Invention
The present invention relates generally to magnetoresistive sensors, but more particularly it relates to an improved thin film vector magnetometer that produces an output voltage which varies linearly with the component of magnetic field in the plane of the thin film, of which it is fabricated, along a predetermined sensitivity axis.
2. Description of the prior art
The detection of magnetic fields is a common function required of many systems. Magnetometers of various types are found in torpedoes, mines, buoys, intrusion detectors, vehicle detectors, and magnetic anomaly detectors to name a few systems. Accordingly, there are wide ranges in the levels, dynamic ranges, and frequency of fields which must be detected. In addition, system level requirements impose constraint such as the maximum power consumption, physical size and operational conditions, e.g., temperature and ambient noise.
Several thin film magnetometers are disclosed in the prior art. In the publication, R. S. Hebbert and L. J. Schwee, "Thin Film Magnetoresistance Magnetometer", The Review Of Scientific Instruments, Vol. 37, No. 10, pp. 1331-1323, October 1966, there is disclosed a magnetometer that uses an etched thin film layer arranged in a Wheatstone bridge configuration. The magnetoresistive properties of the thin film layer are used to provide a resistance change in the arms of the Wheatstone bridge when subjected to a dynamic magnetic field. The dynamic magnetic field produces an imbalance in the arms of the bridge. This imbalance is a function of the intensity of the dynamic magnetic field thereby providing a corresponding output signal amplitude. The thin film layer is arranged apparently with its easy axis aligned orthogonal to a constant hard axis of a bias field. The bias field is applied parallel to the plane of the thin film layer and is of an intensity much greater than the anisotropy field H.sub.k of the thin film layer. This arrangement, due to the fast domain switching characteristics of the thin film layer, has an inherently fast response time with a stated flat response to dynamic magnetic fields of frequencies in the order of 65 MHz.
Paul et al, in U.S. Pat. No. 3,546,579, patented on Dec. 8, 1970, disclose a magnetometer that uses four anisotropic thin films arranged in a Wheatstone bridge configuration. The magnetoresistive properties of the thin films are used to change the resistance in the arms of the bridge when subjected to a dynamic magnetic field. The four thin films are arranged with their easy axes aligned orthogonally to a constant hard axis bias field. The pair of thin films in the balancing legs of the bridge are subjected to bias fields of opposite polarities such that the bridge network is rendered substantially insensitive to ambient or other hard axis field components. Paul et al consider their bias scheme to be an improvement over the bias scheme used by Hebbert and Schwee, aforementioned.
In practice the foregoing magnetometers suffer from several problems. In both the Hebbert et al and Paul et al magnetometers, size and impedance are critical factors in that the devices are larger and have lower impedances than desired. Also, in the Paul et al magnetometer, there is a question of whether there is a practical technique for fabricating their Wheatstone bridge using multiple bias fields. Thus the problem presented is to maintain their self-cancelling feature while producing a magnetometer that is small in physical size.
Consequently, there is a need in the prior art to configure a thin film vector magnetometer that can produce an output voltage which varies linearly with the component of a magnetic field in the plane of the thin film along a predetermined sensitivity axis while maintaining low power consumption and miniaturization of the finished product.
The prior art, as indicated hereinabove, include some advancements in thin film vector magnetometers. However, insofar as can be determined, no prior art thin film vector magnetometer incorporates all of the features and advancements of the present invention.